Process for the oxidation of dialkyl esters of 2-hydroxy-ethylphosphonic acid

ABSTRACT

A process for the preparation of diesters of carboxymethylphosphonic acid of the formula: (RO) 2  P(O)CH 2  CO 2  H, in which R is a straight-chain or branched alkyl radical having 1 to 20 carbon atoms, a cycloaliphatic radical having 5 to 8 carbon atoms, a phenyl or naphthyl radical which can be substituted, an aralkyl radical having 7 to 10 carbon atoms, or in which the (RO) 2  P group forms a ring having 2 to 5 carbon atoms which can be substituted, characterized in that dialkyl ester of 2-hydroxyethylphosphonic acid of the formula (RO) 2  P(O)CH 2  CH 2  OH, in which R has the abovementioned meaning, is reacted with oxygen in the presence of water and a catalyst which contains at least one metal selected from the platinum metals group.

The present invention relates to a process for the preparation ofdialkyl esters of carboxymethylphosphonic acid by oxidation of thecorresponding dialkyl esters of 2-hydroxyethylphosphonic acid withoxygen in the presence of a catalyst.

Dialkyl esters of carboxymethylphosphonic acid can be used as complexingagents or as intermediates in the preparation of organophosphorusproducts.

It is known to prepare diethyl esters of carboxymethylphosphonic acid byheating benzyl chloroacetate with triethylphosphite and subsequenthydrogenolysis of the benzyl ester of the diethyl esters ofcarboxymethylphosphonic acid obtained as an intermediate. Di-n-butylcarboxymethylphosphonate had been prepared in an analogous fashion. (D.J. Martin, C. E. Griffin, J. Org. Chem. 30 [1965] 4034). Diethyl esterof carboxymethylphosphonic acid has also been prepared from thecorresponding ethyl ester by partial hydrolysis with an equimolar amountof KOH in aqueous ethanol. The corresponding dimethyl ester was preparedfrom the methyl ester of dimethyl carboxymethylphosphonates by partialhydrolysis with KOH in methanol, and the corresponding di-n-butyl esterfrom the tributyl ester by partial hydrolysis with aqueous KOH (R. A.Malevannaya et al., Zh. Obshch. Khim. 41 [1971] 1426-1434).

However, in the hydrolysis of triesters of carboxymethylphosphonic acidin alkaline medium, by-products are frequently produced, since, apartfrom the ester group, the P-alkoxy group can also be hydrolyticallycleaved. With the use of KOH, the subsequent work-up leads, moreover, tothe formation of a stoichiometric amount of salt, the removal of whichposes problems.

The aim was therefore to develop a process which delivers dialkyl estersof carboxymethylphosphonic acid in high yield and purity, withoutproducing large amounts of unwanted salt at the same time. The presentinvention now makes such a process available.

The object of the present invention is a process for the preparation ofdialkyl esters of carboxymethylphosphonic acid of the formula (RO)₂P(O)CH₂ CO₂ H, in which R is a straight-chain or branched alkyl radicalhaving 1 to 20 carbon atoms, preferably having 1 to 8 carbon atoms, acycloaliphatic radical having 5 to 8 carbon atoms, a phenyl or naphthylradical which can be substituted, an aralkyl radical having 7 to 10carbon atoms, or in which the (RO)₂ P group forms a ring which contains2 to 5 carbon atoms which can be substituted, characterized in thatdialkyl ester of 2-hydroxyethylphosphonic acid of the formula (RO)₂P(O)CH₂ CH₂ OH, in which R has the above-mentioned meaning, is reactedwith oxygen in the presence of water and a catalyst which contains atleast one metal selected from the platinum metals group.

The substituents on the phenyl or naphthyl radical are preferably one ormore alkyl radicals having 1 to 6 carbon atoms, alkoxy having 1 to 6carbon atoms or halogen, preferably chlorine or bromine. As an alkylradical or cycloaliphatic radical, R can also contain heteroatoms,preferably oxygen or nitrogen.

Metals selected from the platinum metals group which may be mentionedare for example platinum, palladium, iridium, rhodium and ruthenium,palladium and/or platinum being preferred. Particular preference isgiven to catalysts which exclusively contain platinum as the metal ofthe platinum group.

The metals mentioned are preferably applied to a support, in particularactivated charcoal. The weight fraction of the metals in this case isexpediently 1 to 10% of the total weight of the catalyst. Suitablecatalysts are for example commercial catalysts having 5 to 10% by weightof platinum on activated charcoal.

In the process according to the invention, it is expedient to add to thestarting material a quantity of water such that the content of dialkylester of hydroxyethylphosphonic acid in the resulting solution is 5 to30% by weight, preferably 10 to 20% by weight.

The preferred oxidant is pure oxygen. However, mixtures of oxygen withgases inert to the reactants under the reaction conditions canalternatively be used, for example in the form of air, for examplemixtures of oxygen with inert gases or with air.

The total pressure is generally between 0.5 and 100 bar. The reactionvelocity increases markedly with increasing parial [sic] pressure ofoxygen. However, the advantages offered by the increased reactionvelocity at elevated O₂ partial pressure can be at least made up for bythe resulting substantially greater expenditure in terms of apparatus. Atotal pressure of 1 to 10 bar is expedient, employment of atmosphericpressure being particularly simple.

The process according to the invention is generally carried out at atemperature of 30° C. up to the boiling point of the aqueous medium,preferably of 50° to 95° C., in particular 60° to 90° C.

The process according to the invention proceeds in a three-phase systemof solid catalyst, aqueous medium and gaseous oxygen. It can be carriedout in all apparatuses which are suitable for carrying out reactions inthe liquid phase with or without employment of overpressure. Examples ofthis are carrying out the reaction in a stirred vessel or in a bubblecolumn with suspended catalyst. However, the oxidation can also becarried out on a fixed bed with a granular catalyst in a trickle phasereactor.

The reaction time required for the formation of the particular desiredreaction product is expediently determined by removing samples of theliquid reaction mixture at certain periods of time and analyzing them.For example, the yield of reaction product can be continuouslydetermined in a simple manner by analysis of a sample with the aid ofhigh pressure liquid chromatography in comparison to standard solutions.It is recommended to optimize the reaction conditions so that thereaction time is as short as possible, since an unnecessarily prolongedintroduction of oxygen can lead to hyperoxidations, for example todecarboxylations, and thus to a loss in yield of the desired reactionproduct.

The starting materials for the process according to the invention areknown or can be prepared by process procedures known in principle, cf.for example Chelientsev, Kuskov, Zhur. Obsh. Chei Khim., 16, [1946],1481.

The reaction mixture can be worked up by conventional methods. In asuitable process, the water is first removed by distillation. Asubsequent purification is generally not required; the purities aregreater than 90%.

The process according to the invention has the advantage, compared tothe conventional processes mentioned in the introduction, that theformation of unwanted products, such as inorganic salts, is avoided. Inthe catalytic oxidation according to the invention, apart from thedesired products, only water is unavoidably formed, which is present inany case in the reaction medium.

EXAMPLES

1. 80 1 (S.T.P)/h of oxygen at 80° C. were introduced from below througha glass frit into an externally heated vertically arranged glass tube(diameter 50 mm, length 1200 mm), which had been filled with a mixtureof 182 g of diethyl hydroxyethylphosphonate, 728 g of water and 45 g ofa commercial catalyst (5% platinum on activated charcoal). After 9hours, the reaction solution contained 157 g of diethylcarboxymethylphosphonate, corresponding to a yield of 86% of theory.

2. 154 g of dimethyl hydroxyethylphosphonate are reacted for 12 hours at60° C. analogously to Example 1. The reaction mixture contained 160 g ofdimethyl carboxymethylphosphonate, corresponding to a yield of 95% oftheory.

We claim:
 1. A process for the preparation of diesters ofcarboxymethylphosphonic acid of the formula (RO)₂ P(O)CH₂ CO₂ H, inwhich R is a straight-chain or branched alkyl radical having 1 to 20carbon atoms, a cycloaliphatic radical having 5 to 8 carbon atoms, aphenyl or naphthyl radical which can be substituted, an aralkyl radicalhaving 7 to 10 carbon atoms, or in which the (RO)₂ P group forms a ringwhich contains 2 to 5 carbon atoms which can be substituted, whichcomprises: reacting diesters of 2-hydroxyethylphosphonic acid of theformula (RO)₂ P(O)CH₂ CH₂ OH, in which R has the abovementioned meaning,with oxygen in the presence of water and a solid catalyst which containsat least one metal selected from the platinum metals group.
 2. Theprocess according to claim 1, wherein R is an alkyl radical having 1 to8 carbon atoms.
 3. The process according to claim 1, wherein thecatalyst contains platinum and/or palladium.
 4. The process according toclaim 1, wherein said metal consists essentially of platinum.
 5. Theprocess according to claim 1, wherein the catalyst comprises 1 to 10% byweight of at least one metal selected from the platinum metals group anda support material.
 6. The process according to claim 5, wherein saidsupport material comprises activated charcoal.
 7. The process accordingto claim 1, wherein the reaction is carried out in the presence of aquantity of water such that the content of diester ofhydroxyethylphosphonic acid in the resulting solution is 5 to 30% byweight.
 8. The process according to claim 7, wherein said content ofdiester of hydroxyethylphosphonic acid in the resulting solution is 10to 20% by weight.
 9. The process according to claim 1, wherein a totalpressure of 0.5 to 100 bar is employed.
 10. The process according toclaim 9, wherein said total pressure is in the range of 1 to 10 bar. 11.The process according to claim 9, wherein said total pressure isatmospheric pressure.
 12. The process according to claim 1, wherein thereaction is carried out at a temperature of 30° C. up to the boilingpoint of the reaction medium.
 13. The process according to claim 12,wherein said temperature is in the range of 50° to 95° C.
 14. Theprocess according to claim 12, wherein said temperature is in the rangeof 60° to 90° C.
 15. The process according to claim 1, wherein theoxygen is used in a mixture with a gas inert to the reactants under thereaction conditions.